The windshield effect, as I've read about in the context of Roguelikes, is when the player can absently mindedly drift through enemies. This is akin to the passive mass murder of insects many people engage in on a daily basis thanks to the power of windshields. My games, at least starting with Dead Man Walking, have all been guilty of this windshield effect to a substantial degree.
The substantial nature is important. Most Roguelikes I've played have something that could constitute as a "windshield moment". For example, the player might wade through a swarm of giant butterflies rapidly in Dungeon Crawl Stone Soup. This isn't necessarily the most engaging moment of gameplay within DCSS, but it happens rarely and has some tactical implications.
Contrast the DCSS moment with something like the zombies in Dead Man Walking. Zombies are a constant nuisance and every single level of the dungeon is full of them. The least egregious game I've made in regards to windshield moments is probably my most recent release, Lost Valkyrie. There's a pretty large variety of enemies that can spawn on any given floor and thus there's never a swarm of any one particular foe.
However, even Lost Valkyrie suffers a bit: while it avoids repeating the exact monster, the concept of slaughtering large numbers of mobs is still very much alive. Contrast this with some other Roguelikes, like Nethack or Shiren the Wanderer, where the player will encounter a few monsters on each floor, but the threat level of those monsters is considerably higher than the zombies of Dead Man Walking.
One of my goals with Revenge of the Starmen is to embrace the idea of meaningful encounters. I don't want every instance of combat to be a windshield moment. Hell, I don't even want every other instance to be a windshield moment. My goals with this change are to emphasize the importance of exploration, treasure hunting, and careful consideration of each foe. When each floor contains only ~10 mobs instead of ~25 I suspect the player will be more invested in each individual encounter.
In Lost Valkyrie, each room spawned between 1-2 mobs with a small chance to spawn a 3rd. In Revenge of the Starmen, each room spawns between 0-1 mobs with an even smaller chance to spawn a 2nd. Enemies have, of course, been tuned to be much more threatening in general.